1. Field of the Invention
The present invention generally concerns solar energy collection. More specifically, the present invention concerns increasing the efficiency of a tubular solar collector apparatus.
2. Description of the Related Art
The application of solar energy to heating generally requires a collector that efficiently absorbs solar radiation. The collector transfers the radiated energy to a fluid, which transports the energy to a final application in the form of heat. This final application may include a domestic water or space heating apparatus. An effective collector must absorb a high percentage of incident solar radiation, while losing only a small amount of the absorbed energy to the ambient through either heat conduction or radiation.
Solar collectors including two concentric glass tubes with an evacuated space there between have generally been recognized as an effective configuration for absorbing a high percentage of incident radiation and minimizing heat loss by conduction. A solar collector configured in such a manner is similar to the configuration of a Dewar flask, which may be used as an insulated storage vessel, and are sometimes appropriately referred to as Dewar-type evacuated-tube collectors. The need to minimize radiation heat loss has been addressed by coating the vacuum-side of the inner glass tube with a selective surface that has a high absorptivity for visible radiation and low emissivity for infrared radiation.
The primary developmental effort relating to evacuated tube solar concerns removal of the thermal energy absorbed by the elongated glass tubes. One method of removing the thermal energy from the elongated inner glass tube of evacuated tube solar collectors is to circulate water or other working fluid into and out of the interior of the glass tube. The working fluid circulated through the glass tube absorbs the solar energy and carries that energy to a location where the energy can be stored or put to practical use. An alternative methodology circulates the water or working fluid through the elongated glass tube via pipes or circulation tubes positioned inside the glass tube so that the water or other working fluid does not actually come in contact with the glass tube.
A further technique uses heat pipes to transfer the absorbed solar energy to a working fluid medium that functions as a heat sink. The heat sink “stores” the collected thermal energy and/or transfers the energy to a location where the stored energy can be put to practical use. In such an embodiment, the heat pipe may include an evaporator portion that absorbs the solar energy and causes a volatile thermal transfer fluid in the heat pipe—not the working fluid medium—to vaporize. The vapor pressure drives the vapor toward the cooler condenser section of the heat pipe, which is placed in contact with the working fluid medium or heat sink.
The thermal energy absorbed from the sun in the evaporator portion is conducted from the vapor of the thermal transfer fluid inside the heat pipe to the working fluid or heat sink outside the heat pipe by way of the condenser. The lower temperature of the thermal transfer fluid vapor, which is due to conduction of the heat from the vapor to the working fluid, results in condensation of the thermal transfer fluid in the heat pipe. The condensed thermal transfer fluid then flows from the condenser portion back to the evaporator portion of the heat pipe where solar energy is absorbed to continue the cycle.
An additional method for transferring heat out of a Dewar-type evacuated-tube solar collector involves absorbed solar energy boiling water within the collector. The steam generated from the boiling water transports heat out of the collector through a process called vapor-phase pumping. A solar collector utilizing vapor-phase-pumping involves a tubular absorber filled almost to the top with a heating liquid, such as water, to provide a relatively small vapor-phase zone at the upper end of the absorber; a boiler mounted at a higher elevation than the solar collector; a tube through which liquid flows from the boiler into the tubular absorber and that extends into the interior of the tubular absorber for substantially the full length of the absorber; and a tube that connects the upper vapor-phase zone in the boiler with the vapor-phase zone in the tubular absorber. Extracting heat from an evacuated-tube solar collection using vapor-phase-pumping is easier than with a heat pipe and also avoids the need for a mechanical pump.
It has generally been viewed as disadvantageous not to fill the tubular absorbers such that the top half is in contact with the liquid. Vapor-phase pumping devices have intentionally avoided such a configuration by filling the inner absorber cylinder with liquid or converting the inner absorber cylinder into a heat pipe. Another option has involved inserting a separate heat pipe or U-tube into the inner absorber cylinder and using a metal, thermally conductive fin to thermally couple the evaporator of the heat pipe or the U-tube to the inner absorber cylinder.
The vapor-phase-pumping arrangement described above, however, has several limitations. By operating with the tubular absorber filled almost to the top, the hot fluid within the tubular absorber stores a significant amount of thermal energy. Most of this thermal energy will be lost to the ambient during the night. Furthermore, if the fluid within the tubular absorber is water, the absorber is likely to be damaged by freezing in cold climates since water expands when it freezes. If the vapor that is produced within the absorber is to flow to the boiler without interfering with the in-flowing liquid, an inlet tube that extends into the absorber must be used. This tube, which is often metallic as to withstand possible stagnation conditions within the evacuated-tube collector, increases the cost for the solar collector, especially for more expensive metals such as copper.